Immunocompetence is the ability of the body to produce a normal immune response (i.e., antibody production and/or cell-mediated immunity) following exposure to a pathogen, which might be a live organism (such as a bacterium or fungus), a virus, or specific antigenic components isolated from a pathogen and introduced in a vaccine. Immunocompetence is the opposite of immunodeficiency or immuno-incompetent or immunocompromised. Several examples would be a newborn that does not yet have a fully functioning immune system but may have maternally transmitted antibody (immunodeficient); a late stage AIDS patient with a failed or failing immune system (immuno-incompetent); a transplant recipient taking medication so their body will not reject the donated organ (immunocompromised); age-related attenuation of T cell function in the elderly; or individuals exposed to radiation or chemotherapeutic drugs. There may be cases of overlap but these terms are all indicators of a dysfunctional immune system. In reference to lymphocytes, immunocompetence means that a B cell or T cell is mature and can recognize antigens and allow a person to mount an immune response.
Immunocompetence depends on the ability of the adaptive immune system to mount an immune response specific for any potential foreign antigens, using the highly polymorphic receptors encoded by B cells (immunoglobulins, Igs) and T cells (T cell receptors, TCRs).
Igs expressed by B cells are proteins consisting of four polypeptide chains, two heavy chains (H chains) and two light chains (L chains), forming an H2L2 structure. Each pair of H and L chains contains a hypervariable domain, consisting of a VL and a VH region, and a constant domain. The H chains of Igs are of several types, μ, δ, γ, α, and β. The diversity of Igs within an individual is mainly determined by the hypervariable domain. The V domain of H chains is created by the combinatorial joining of three types of germline gene segments, the VH, DH, and JH segments. Hypervariable domain sequence diversity is further increased by independent addition and deletion of nucleotides at the VH-DH, DH-JH, and VH-JH junctions during the process of Ig gene rearrangement. In this respect, immunocompetence is reflected in the diversity of Igs.
TCRs expressed by αβ T cells are proteins consisting of two transmembrane polypeptide chains (α and β), expressed from the TCRA and TCRB genes, respectively. Similar TCR proteins are expressed in gamma-delta T cells, from the TCRD and TCRG loci. Each TCR peptide contains variable complementarity determining regions (CDRs), as well as framework regions (FRs) and a constant region. The sequence diversity of αβ T cells is largely determined by the amino acid sequence of the third complementarity-determining region (CDR3) loops of the α and β chain variable domains, which diversity is a result of recombination between variable (Vβ), diversity (Dβ), and joining (Jβ) gene segments in the β chain locus, and between analogous Vα and Jα gene segments in the α chain locus, respectively. The existence of multiple such gene segments in the TCR α and β chain loci allows for a large number of distinct CDR3 sequences to be encoded. CDR3 sequence diversity is further increased by independent addition and deletion of nucleotides at the Vβ-Dβ, Dβ-Jβ, and Vα-Jα junctions during the process of TCR gene rearrangement. In this respect, immunocompetence is reflected in the diversity of TCRs.
There exists a long-felt need for methods of assessing or measuring the adaptive immune system of patients in a variety of settings, whether immunocompetence in the immunocompromised, or dysregulated adaptive immunity in autoimmune disease. A demand exists for methods of diagnosing a disease state or the effects of aging by assessing the immunocompetence of a patient. In the same way results of therapies that modify the immune system need to be monitored by assessing the immunocompetence of the patient while undergoing the treatment. Conversely, a demand exists for methods to monitor the adaptive immune system in the context of autoimmune disease flares and remissions, in order to monitor response to therapy, or the need to initiate prophylactic therapy pre-symptomatically.